How to merge two sculpted meshes?
You may use a Direct Union option of a BoolTool add on.

Activate it in User Preferences (Ctrl+Alt+U).


In my case I have four objects with different vertex count. Some of them were sculpted, some of them not.


In Object Mode select all the objects and press Shift+Ctrl+Num+ (or press Spacebar and type 'direct'-->Direct Union). 


After this operation you'll achieve a joined mesh ready for sculpting.